The genetic control of gonad maturation is studied in two species of poeciliid fish, Xiphophorus pygmaeus and X. maculatus. Genetic crosses are in progress to ascertain the number of alleles involved and their mode of inheritance. Physiological studies are in progress to determine the site of action of these genes; 1) Measuring the growth rates of the different genotypes from the day of birth and recording the age, size and weight when the fish initiate sexual maturity; 2) Cytology of the pituitary gland of the different genotypes; 3) using special homozygous stocks and their hybrids to avoid histoincompatibility, pituitary transplants are made from genetically "early" genotypes into genetically "late" recipients.